scholarly journals The Impact of Variable Inlet Mixture Stratification on Flame Topology and Emissions Performance of a Premixer/Swirl Burner Configuration

2012 ◽  
Vol 2012 ◽  
pp. 1-12 ◽  
Author(s):  
P. Koutmos ◽  
G. Paterakis ◽  
E. Dogkas ◽  
Ch. Karagiannaki
Keyword(s):  
Vortex Formation ◽  
Gas Analysis ◽  
Large Eddy Simulations ◽  
Mixing Conditions ◽  
Swirl Burner ◽  
Large Eddy ◽  
Air Mixing ◽  
Set Up ◽  
The Impact ◽  
Plane Disk

The work presents the assessment of a low emissions premixer/swirl burner configuration utilizing lean stratified fuel preparation. An axisymmetric, single- or double-cavity premixer, formed along one, two, or three concentric disks promotes propane-air premixing and supplies the combustion zone at the afterbody disk recirculation with a radial equivalence ratio gradient. The burner assemblies are operated with a swirl co-flow to study the interaction of the recirculating stratified flame with the surrounding swirl. A number of lean and ultra-lean flames operated either with a plane disk stabilizer or with one or two premixing cavity arrangements were evaluated over a range of inlet mixture conditions. The influence of the variation of the imposed swirl was studied for constant fuel injections. Measurements of turbulent velocities, temperatures, OH* chemiluminescence and gas analysis provided information on the performance of each burner set up. Comparisons with Large Eddy Simulations, performed with an 11-step global chemistry, illustrated the flame front interaction with the vortex formation region under the influence of the variable inlet mixture stratifications. The combined effort contributed to the identification of optimum configurations in terms of fuel consumption and pollutants emissions and to the delineation of important controlling parameters and limiting fuel-air mixing conditions.

High Resolution Large Eddy Simulations to Evaluate Turbulence Properties Within a Real Helicopter Engine Combustor

2018 ◽  
Author(s):  
Charlie Koupper ◽  
Jean Lamouroux ◽  
Stephane Richard ◽  
Gabriel Staffelbach
Keyword(s):  
Gas Turbine ◽  
Turbulence Intensity ◽  
Numerical Scheme ◽  
Mesh Refinement ◽  
Large Eddy Simulations ◽  
Scale Model ◽  
Large Eddy ◽  
Set Up ◽  
High Level ◽  
The Impact

In a gas turbine, the combustor is feeding the turbine with hot gases at a high level of turbulence which in turns strongly enhances the heat transfer in the turbine. It is thus of primary importance to properly characterize the turbulence properties found at the exit of a combustor to design the turbine at its real thermal constraint. This being said, real engine measurements of turbulence are extremely rare if not inexistent because of the harsh environment and difficulty to implement experimental techniques that usually operate at isothermal conditions (e.g. hot wire anemometry). As a counterpart, high fidelity unsteady numerical simulations using Large Eddy Simulations (LES) are now mature enough to simulate combustion processes and turbulence within gas turbine combustors. It is thus proposed here to assess the LES methodology to qualify turbulence within a real helicopter engine combustor operating at take-off conditions. In LES, the development of turbulence is primarily driven by the level of real viscosity in the calculation, which is the sum of three contributions: laminar (temperature linked), turbulent (generated by the sub-grid scale model) and artificial (numerics dependent). In this study, the impact of the two main sources of un-desired viscosity is investigated: the mesh refinement and numerical scheme. To do so, three grids containing 11, 33 and 220 million cells for a periodic sector of the combustor are tested as well as centred second (Lax-Wendroff) and third order (TTGC) in space schemes. The turbulence properties (intensity and integral scales) are evaluated based on highly sampled instantaneous solutions and compared between the available simulations. Results show first that the duration of the simulation is important to properly capture the level of turbulence. If short simulations (a few combustor through-times) may be sufficient to evaluate the turbulence intensity, a bias up to 14% is introduced for the turbulence length scales. In terms of calculation set-up, the mesh refinement is found to have a limited influence on the turbulence properties. The numerical scheme influence on the quantities studied here is small, highlighting that the employed schemes dissipation properties are already sufficient for turbulence characterization. Finally, spatially averaged values of turbulence intensity and lengthscale at the combustor exit are almost identically predicted in all cases. However, significant variations from hub to tip are reported, which questions the pertinence to use 0-D turbulence boundary conditions for turbines. Based on the set of simulations discussed in the paper, guidelines can be derived to adequately set-up (mesh, scheme) and run (duration, acquisition frequency) a LES when turbulence evaluation is concerned. As no experimental counterpart to this study is available, the conclusions mainly aim at knowing the possible numerical bias rather than commenting on the predictivity of the approach.

Geometry Effects on Thermal Striping in Nuclear Reactors: POD Analysis of Large-Eddy Simulations and Experiments

2017 ◽  
Author(s):  
Oana Marin ◽  
Elia Merzari ◽  
Aleks Obabko ◽  
Andres Alvarez ◽  
Stephen Lomperski ◽  
...  
Keyword(s):  
Nuclear Reactor ◽  
Flow Behavior ◽  
National Laboratory ◽  
Argonne National Laboratory ◽  
Large Eddy Simulations ◽  
Large Eddy ◽  
Set Up ◽  
Pod Analysis ◽  
Thermal Striping ◽  
The Impact

Thermal striping is of particular significance in nuclear reactor applications, primarily in sodium cooled fast reactors. The mixing chamber of the upper plenum of a nuclear reactor can be subjected to thermal striping unless designed such that the coolant is sufficiently mixed prior to reaching the top wall of the upper plenum. In order to conduct a systematic analysis of this phenomenon a simplified experimental set-up was designed and built at Argonne National Laboratory. In a parallel effort a similar simulation was conducted using the spectral-element code Nek5000. The set-up consists of two turbulent jets entering a rectangular tank via two hexagonal inlets, the interesting phenomena being the mixing within the tank. Two different inlet geometries were studied previously, both experimentally and via high-fidelity large-eddy simulations reporting various turbulent statistical quantities. To further assess the flow behavior we hereby perform a Proper Orthogonal Decomposition (POD) to identify the most dominant energetic modes and quantify their impact on the top wall of the upper plenum. The POD analysis of the experimental data in both inlet geometrical configurations is compared with LES and presented to highlight the impact of geometry on the velocity and thermal fields. We find a qualitative coherence between both simulation and experiment, characterized by a strong backflow in the weakly stable geometry, as indicated by the first mode, and the presence of three stagnation points in the strongly stable geometry setup. Also we identify a pairing of modes 1 and 3 with higher frequency than the second mode. This pairing is opposite in the two flow configurations leading to a faster decay of one of the jets in one case and a stable flow in the other.

Prediction of liquid-liquid flow in an SMX static mixer using large eddy simulations

2010 ◽  
Vol 64 (2) ◽  
Author(s):  
Paulina Pianko-Oprych ◽  
Zdzisław Jaworski
Keyword(s):  
Centrifugal Force ◽  
Liquid Flow ◽  
Concentration Distribution ◽  
Large Eddy Simulations ◽  
Dispersed Phase ◽  
Static Mixer ◽  
Phase Concentration ◽  
Large Eddy ◽  
Smx Static Mixer ◽  
The Impact

AbstractThe main purpose of the paper is to apply the large eddy simulations (LES) technique and to verify its use as a predicting tool for turbulent liquid-liquid flow in an SMX static mixer. LES modeling was carried out using the Smagorinsky-Lilly model of the turbulent subgrid viscosity for the Reynolds number of 5000 and 10000. The continuous phase was water and the dispersed phase was silicon oil. The investigation covers the effects of the density ratio between the phases. Three different cases of liquid densities were considered. The dispersed phase concentration distribution in the mixer cross-sections was compared with the corresponding time averaged results obtained formerly for the same configuration in a steady-state simulation using the standard RANS approach with the k-ɛ model. The dependency of the standard deviation of the dispersed phase concentration on the distance from the mixer inlet and the impact of the centrifugal force on the phase concentration distribution were investigated. The presented results for the SMX static mixer confirm conclusions of previous studies by Jaworski et al. (2006) obtained for a Kenics static mixer and show less a pronounced influence of the centrifugal force on the phase concentration distribution of the LES results in comparison to the RANS case.

scholarly journals High-resolution large-eddy simulations of sub-kilometer-scale turbulence in the upper troposphere lower stratosphere

2013 ◽  
Vol 13 (12) ◽  
pp. 31891-31932 ◽  
Author(s):  
R. Paoli ◽  
O. Thouron ◽  
J. Escobar ◽  
J. Picot ◽  
D. Cariolle
Keyword(s):  
Large Scale ◽  
Lower Stratosphere ◽  
Atmospheric Model ◽  
Large Eddy Simulations ◽  
Stratified Flows ◽  
Flow Topology ◽  
Upper Troposphere ◽  
Large Eddy ◽  
Scale Turbulence ◽  
The Impact

Abstract. Large-eddy simulations of sub-kilometer-scale turbulence in the upper troposphere lower stratosphere (UTLS) are carried out and analyzed using the mesoscale atmospheric model Méso-NH. Different levels of turbulence are generated using a large-scale stochastic forcing technique that was especially devised to treat atmospheric stratified flows. The study focuses on the analysis of turbulence statistics, including mean quantities and energy spectra, as well as on a detailed description of flow topology. The impact of resolution is also discussed by decreasing the grid spacing to 2 m and increasing the number of grid points to 8×109. Because of atmospheric stratification, turbulence is substantially anisotropic, and large elongated structures form in the horizontal directions, in accordance with theoretical analysis and spectral direct numerical simulations of stably stratified flows. It is also found that the inertial range of horizontal kinetic energy spectrum, generally observed at scales larger than a few kilometers, is prolonged into the sub-kilometric range, down to the Ozmidov scales that obey isotropic Kolmorogov turbulence. The results are in line with observational analysis based on in situ measurements from existing campaigns.

Can Water Dilution Avoid Flashback On a Hydrogen Enriched Micro Gas Turbine Combustion? – A Large Eddy Simulations Study

2021 ◽  
Author(s):  
Alessio Pappa ◽  
Laurent Bricteux ◽  
Pierre Bénard ◽  
Ward De Paepe
Keyword(s):  
Reaction Rates ◽  
Hydrogen Combustion ◽  
Operating Conditions ◽  
Large Eddy Simulations ◽  
Reference Case ◽  
Large Eddy ◽  
Pure Methane ◽  
The Impact ◽  
Water Dilution ◽  
Air Humidification

Abstract Considering the growing interest in Power-to-Fuel, i.e. production of H2 using electrolysis to store excess renewable electricity, combustion-based technologies still have a role to play in the future of power generation. Hydrogen combustion is well-known to lead to combustion instabilities. The high temperatures and reaction rates can potentially lead to flashback. In the past, combustion air humidification has proven effective to reduce temperatures and reaction rates. Therefore, humidification can open a path to stabilize hydrogen combustion. However, accurate data assessing the impact of humidification on the combustion is still missing for real mGT combustor geometries and operating conditions. This paper presents a comparison between pure methane and hydrogen enriched methane/air combustions, with and without air humidification, in a typical mGT combustion chamber (Turbec T100) using Large Eddy Simulations analysis. In a first step, the necessary minimal water dilution, to reach stable combustion with hydrogen, was assessed using a 1D approach. The one-dimensional unstretched laminar flame is computed for both pure methane (reference case) and hydrogen enriched cases. The results of this comparison show that the same level of flame speed as in the reference case can be reached by adding 10% (in mass fraction) of water. In a second step, high fidelity LES on the 3D geometry are performed to show that water dilution helped to lower the temperature and reaction rate of hydrogen at same levels as reference case, and thus prevents flashback, enabling the use of hydrogen blends in the mGT.

Large-Eddy Simulations of Heat Transfer Within a Multi-Perforation Synthetic Jets Configuration

2019 ◽  
Author(s):  
Soizic Esnault ◽  
Florent Duchaine ◽  
Laurent Gicquel
Keyword(s):  
Heat Transfer ◽  
Flow Behavior ◽  
Synthetic Jets ◽  
Large Eddy Simulations ◽  
Heated Plate ◽  
Piston Displacement ◽  
Large Eddy ◽  
Grazing Flow ◽  
The Impact ◽  
Ejection Phase

Abstract Synthetic jets are produced by devices that enable a suction phase followed by an ejection phase. The resulting mean mass budget is hence null and no addition of mass in the system is required. These particular jets have especially been considered for some years for flow control applications. They also display features that can become of interest to enhance heat exchanges, for example for wall cooling issues. Synthetic jets can be generated through different mechanisms, such as acoustics by making use of a Helmholtz resonator or through the motion of a piston as in an experience mounted at Institut Pprime in France. The objective of this specific experiment is to understand how synthetic jets can enhance heat transfer in a multi-perforated configuration. As a complement to this experimental set up, Large-Eddy Simulations are produced and analysed in the present document to investigate the flow behavior as well as the impact of the synthetic jets on wall heat transfer. The experimental system considered here consists in a perforated heated plate, each perforation being above a cavity where a piston is used to control the synthetic jets. Placed in a wind tunnel test section, the device can be studied with a grazing flow and multiple operating points are available. The one considered here implies a grazing flow velocity of 12.8 m.s−1, corresponding to a Mach number around 0.04, and a piston displacement of 22 mm peak-to-peak at a frequency of 12.8 Hz. These two latter parameters lead to a jet Reynolds number of about 830. A good agreement is found between numerical results and experimental data. The simulations are then used to provide a detailed understanding of the flow. Two main behaviours are found, depending on the considered mid-period. During the ejection phase, the flow transitions to turbulence and the formation of characteristic structures is observed; the plate is efficiently cooled. During the suction phase the main flow is stabilised; the heat enhancement is particularly efficient in the hole wakes but not between them, leading to a heterogeneous temperature field.

scholarly journals Large Eddy Simulations with Conjugate Heat Transfer (CHT) modeling of Internal Combustion Engines (ICEs)

2019 ◽  
Vol 74 ◽  
pp. 51 ◽  
Author(s):  
Angela Wu ◽  
Seunghwan Keum ◽  
Volker Sick
Keyword(s):  
Heat Transfer ◽  
Surface Temperature ◽  
Wall Temperature ◽  
Conjugate Heat Transfer ◽  
Internal Combustion Engines ◽  
Internal Combustion ◽  
Large Eddy Simulations ◽  
Temperature Fields ◽  
Large Eddy ◽  
The Impact

In this study, the effects of the thermal boundary conditions at the engine walls on the predictions of Large-Eddy Simulations (LES) of a motored Internal Combustion Engine (ICE) were examined. Two thermal boundary condition cases were simulated. One case used a fixed, uniform wall temperature, which is typically used in conventional LES modeling of ICEs. The second case utilized a Conjugate Heat Transfer (CHT) modeling approach to obtain temporally and spatially varying wall temperature. The CHT approach solves the coupled heat transfer problem between fluid and solid domains. The CHT case included the solid valves, piston, cylinder head, cylinder liner, valve seats, and spark plug geometries. The simulations were validated with measured bulk flow, near-wall flow, surface temperature, and surface heat flux. The LES quality of both simulations was also discussed. The CHT results show substantial spatial, temporal, and cyclic variability of the wall heat transfer. The surface temperature dynamics obtained from the CHT model compared well with measurements during the compression stroke, but the absolute magnitude was 5 K (or 1.4%) off and the prediction of the drop in temperature after top dead center suffered from temporal resolution limitations. Differences in the predicted flow and temperature fields between the uniform surface temperature and CHT simulations show the impact of the surface temperature on bulk behavior.

scholarly journals Wildfires as a source of airborne mineral dust – Revisiting a conceptual model using Large-Eddy simulations (LES)

2018 ◽  
Author(s):  
Robert Wagner ◽  
Michael Jähn ◽  
Kerstin Schepanski
Keyword(s):  
Conceptual Model ◽  
Mineral Dust ◽  
Large Eddy Simulations ◽  
Dust Particles ◽  
Strong Increase ◽  
Dust Emissions ◽  
Large Eddy ◽  
Fire Area ◽  
Fire Properties ◽  
The Impact

Abstract. Airborne mineral dust is a key player in the Earth system and shows manifold of impacts on atmospheric properties such as the radiation budget and cloud micro-physics. Investigations of smoke plumes originating from wildfires found significant fractions of mineral dust within these plumes – raised by strong turbulent winds related to the fire. The present study revisits the conceptual model describing the emission of mineral dust particles during wildfires by pyro-convection as described by the literature. This is achieved by means of high resolved Large-Eddy simulations (LES), conducted with the All Scale Atmospheric Model (ASAM). The impact of different fire properties representing typical grassland and shrubland fires, and different ambient atmospheric conditions on the fire-driven winds and their capability to mobilize mineral dust particles were investigated. Results from this study illustrate that the energy release of the fire leads to a strong increase in strength and frequency of occurrence of intense near-surface winds, which exceed typical threshold velocities inevitably required for dust emissions. The fire-induced modulations of the wind field can be transported up to some kilometers downstream of the fire area and are able to favor dust emissions also in some distance to the fire area. Although measurements showed already the importance of wildfires on dust emissions, pyro-convection is so far neglected as a dust emission process in atmosphere-aerosol models. The results presented in this study can be seen as the first step towards a systematic parameterization representing the connection between typical fire properties and related dust emissions, which eventually can be implemented in larger-scale aerosol models ultimately contributing to the reduction of uncertainties in the aerosol-climate feedback.

Reliable and Accurate Prediction of Three-Dimensional Separation in Asymmetric Diffusers Using Large-Eddy Simulation

2009 ◽  
Author(s):  
Hayder Schneider ◽  
Dominic von Terzi ◽  
Hans-Jo¨rg Bauer ◽  
Wolfgang Rodi
Keyword(s):  
Experimental Data ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Pressure Coefficient ◽  
Three Dimensional ◽  
Large Eddy Simulations ◽  
Navier Stokes ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Impact

Reynolds-Averaged Navier-Stokes (RANS) calculations and Large-Eddy Simulations (LES) of the flow in two asymmetric three-dimensional diffusers were performed. The numerical setup was chosen to be in compliance with previous experiments. The aim of the present study is to find the least expensive method to compute reliably and accurately the impact of geometric sensitivity on the flow. RANS calculations fail to predict both the extent and location of the three-dimensional separation bubble. In contrast, LES is able to determine the amount of reverse flow and the pressure coefficient within the accuracy of experimental data.

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